Pulmonary arterial hypertension (PAH) is a fatal disease associated with increased pulmonary artery pressure (PAP) and abnormalities in blood vessel growth. The excessive growth of the vascular endothelial cells is the focus of our research. Four years ago, we established methods for culture of pulmonary artery endothelial cells (PAEC) from PAH and donor lungs. The PAEC in culture reflect abnormalities found in PAH lungs and patients in vivo, and made it possible to test our hypotheses in translational mechanistic studies. We found that PAH PAEC have enhanced proliferation & survival, which is dependent on activation of signal transducer and activator of transcription-3 (STAT3), a fundamental regulator of cell survival and angiogenesis. The PAH cells have impaired production of the vasodilator nitric oxide (NO) due to post- translational mechanisms that include phosphorylation/inhibition of endothelial NO synthase (eNOS). In association with low NO, the cells have reduced mitochondria numbers & respiration, which is accompanied by ~3-fold increase in glycolysis for energy production. The shift to anaerobic glycolytic metabolism is related to expression of the pro-survival and pro-angiogenic signal transducer, hypoxia-inducible factor -1a (HIF-1a). Physiologic effects classically ascribed to HIF-expression [increased lung glucose uptake by 18F- fluoro-deoxy-glucose analogue - positron emission tomography (FDG-PET), high-levels of plasma erythropoietin (Epo) and mobilization of bone marrow progenitors] are present in patients and quantitatively associated with PAP. The HIF-expression is mechanistically linked to the alterations in STAT3, NO and Mn superoxide dismutase (MnSOD). Preliminary data show that the PAH PAEC secrete the potent growth- stimulatory & bone marrow-mobilizing hepatocyte growth factor (HGF). Present at high-levels in patient plasma and PAH cell cultures, HGF signals through STAT3 and induces HIF-1a, and is itself induced by STAT3 and increased by hypoxia, which endows the cells with autocrine and paracrine proliferative and pro-angiogenic effects. Here, we hypothesize that PAH endothelial cells have decreased NO production by eNOS and activation of survival pathways STAT3 and HIF-1a. These biochemical mechanisms account for the proliferative, glycolytic, and strongly pro-angiogenic phenotype of the cells. We identify mechanisms of reduced eNOS activity in aim 1, and focus on HIF-expression and mechanisms accounting for its expression in aim 2. In aim 3, we test if HIF-expression is mechanistically important in PAH pathophysiology using the primary cell culture model, a xenograft model of PAH cells in immunodeficient mice and a longitudinal clinical study, in which we test whether experimental outcomes of HIF-effects are associated with clinical outcomes in patients over time. Overall our goals are to define the pathophysiology of the abnormal vascular growth in PAH, and in so doing, apply the knowledge to improve the care of patients.